Seems like he's almost 4.5. Sounds like it's one of those things that you don't exactly want to punish, but you need to correct before it turns into bedtime struggles and midnight fights. Is he being loud and disruptive, or does he just kinda start in the fort and end up rolling around on the floor goofing off quietly for a while? You could make the fort 'disappear' one day and then just tell him the only place left to sleep is the bed. He might fight it for a night or two. Or you could offer a reward for each morning that he wakes up in his own bed. Those are just some ideas.